Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic and mutanogenic pollutants ubiquitous in the natural world. Human activities, including the combustion of fossil fuels, contribute to the total abundance of these compounds in the environment. Semi-volatile PAHs adsorbed to fine particulate matter enter the body via inhalation. It is a matter of public health to understand the fate of PAHs after they are released into the environment. Sunlight-induced photooxidation is an important method of PAH removal from the environment. The proposed research is designed to measure photochemical rate constants and product quantum yields necessary to predict the rates of PAH decay in condensed phase systems including atmospheric particles, water, and other environmental surfaces. Unlike previous studies, the results of this study can be used to identify and quantify PAH photooxidation products in local environments. This is important information because PAH photooxidation products are not only more reactive than PAHs and another group of suspected carcinogens, but they are also more soluble in aqueous systems and consequently can be transported over longer distances. The hypothesis of the proposed work is that PAHs that degrade predominately via the radical-cation mediated mechanism will have degradation rates that are dependent on the electron-donor capacity of the media in which they reside, and there will be a diversity of under-characterized and previously unidentified photoproducts created in these processes. To explore this hypothesis, three specific objectives for the proposed research have been formulated: (1) Characterize photoproducts of PAH photodegradation using structurally sensitive HPLC/MS/MS techniques;(2) Measure formation rates and quantum yields of the observed photoproducts in solutions with varied polarity to elucidate the role of electron donors on the rates of PAH photodegradation in the environment;(3) Manipulate mechanism-specific intermediates during photolysis to elucidate the dominant mechanism for PAH photodegradation in matrices with a range of electron-scavenging potentials. The proposed work will use recently developed LC/MS/MS techniques to measure the product quantum yields of PAH photooxidation in solutions with exceptional sensitivity and selectivity relative to other analytical techniques. The success of this project will help to elucidate the role of the environment on the damaging effects of particle inhalation. The study will also provide kinetic parameters necessary to model the photodegradation of environmental PAHs, inspiring ideas for removal from contaminated sites.